This invention relates to blood clot filters of the permanent metallic type, that may be placed in the inferior vena cava, one of the two main veins for returning blood to the heart. Their purpose is to stop large clots or thrombi from traveling, typically from the leg veins, through the inferior vena cava, to the heart. If such clots reach the lungs, they would occlude the flow of blood and possibly lead to death.
Filters of this type are typically formed of fine wire legs attached to a head or nose cone. The wire legs have a conical aspect in order to channel emboli (or clots) toward the center of the filter, to the nose cone near the apex of the filter. The filter thus may trap clots of approximately three millimeters or larger. The clot, depending on its age and nature, may stay permanently in the filter or may be resorbed by the body.
One blood clot filter of particular effectiveness is a prior commercial embodiment of Kimmell U.S. Pat. No. 3,952,747, sold by Medi-Tech Incorporated of Watertown, Mass., under the trademark GREENFIELD.RTM.. The prior commercial Kimmell filter has stainless steel wire legs extending from a large head. According to Kimmell, the legs are arranged in a conical aspect, each leg bent to form a number of linear segments generally tangent about the conical aspect to increase the filtering effect. When the filter is inserted into a blood vessel, the head and the apex of the cone are positioned downstream in the blood flow. The remote ends of the legs are positioned upstream in the blood flow and are engaged with the vessel wall.
Blood vessel filters have previously been introduced into a vessel surgically, by cutting down to and then into a vein, using surgical blades. The filter, confined within a metallic capsule, has been introduced through a catheter. Once in position, the filter has been dislodged from the capsule using a pusher, and the capsule and attached catheter removed. Typically, the surgical procedure has required two special teams of physicians, including a surgeon and a radiologist, and it is not uncommon for the procedure to take up to 2 hours.
More recently, certain filters have been inserted percutaneously. The advantages of this technique include reduced trauma and shortened operating time. The applicability of this technique has been limited in the case of the preferred Kimmell type of filter.